1. Field of the Invention
This invention relates to a novel high-sensitivity thermosensitive multilayer film and a method for the production of a stencil sheet using the film. More particularly, this invention relates to a high-sensitivity thermosensitive multilayer film suitable for the production of a stencil sheet for recording therein information on a given original by being perforated with the image or lettering of the information at a prescribed position by direct heating with the pulsating irradiation using a laser beam, particularly a semiconducting laser beam of low energy, or a LED beam, indirect heating owing to the steps of heating a given original by flashing instantaneous irradiation using a flash lamp or a xynon-krypton lamp and transferring the heat to the film, or contact conduction of heat with a thermal head and to a method for the production of the stencil sheet mentioned above.
2. Description of the Prior Art
Recently, the technique of preparing a stencil sheet perforated with the information of a given original by means of a thermal head, an electric discharge device, or an electrifying recording device and using this perforated stencil sheet as a plate for printing has been developed and put to use. Studies on new plate-making means relying on the use of a LED array, a laser array, and a laser head, for example, are now underway.
As a means of making the plate of the type mentioned above, the method which comprises tightly superposing on the film surface of the stencil sheet an original having lettering or an image expressed thereon as with ink or toner capable of absorbing a ray, causing the lettering or image on the original to generate heat as by perforating means of high energy region resorting to infrared ray or flash light from the xenon lamp, and conducting the heat to the film surface thereby melting the film for necessary perforation and consequently obtaining the effecting necessary plate making has been known to the art.
As the stencil sheet to be used, in this case for the plate making, the laminate produced by stretching a polyethylene terephthalate film in an independent form and about 2 .mu.m in thickness with an extremely expensive high-precision tenter and, at the same time, thoroughly heat-treating the film in the posterior part of the same tenter thereby crystallizing the resin of the film (to crystallinity of about 45% as determined by the density method) and superposing the resultant stretched film on a supporting member such as onion paper or sheet (in a meshed form) by the use of an adhesive agent in a separate step has been known in the art.
Though this laminate possesses sufficient resistance to heat and exhibits highly satisfactory dimensional stability, it inevitably sustains scratches, wrinkles, warp, strain (uneven tention), and breakage because, during the aforementioned super-position of the film on the supporting member through the medium of the adhesive agent, the adhesion of the supporting member to the film occurs with the irregularities of surface of the supporting member persisting and manifesting themselves through the film surface to the extent of jeopardizing the microscopic smoothness of the film surface and disrupting the evenness of thickness of the film due to uneven application of the adhesive agent along the interface. Moreover, the superposition of one such polyethylene terephthalate film on the supporting member is carried out in a stretched state with meticulous care. In spite of the elaboration, it is difficult to obtain the laminate with fully satisfactory uniformity. The stencil sheet currently obtainable by this lamination, therefore, possesses no fully satisfactory quality.
When the plate for printing is made of the stencil sheet which has inferior quality due to the performance of the laminating method, it entails disadvantages such as the phenomenon of curling the separation of layers along the interface of adhesion, and the heavy local deterioration of sensitivity or resolution. When the perforating means of high energy region is used, this problem does not require very serious consideration. The problem, however, dictates very serious consideration when the perforating means of low energy region is used.
The thermal heads provides with very small heating elements (10 to 16 dots/mm, for example) and utilized extensively in various types of printers, word processors, and facsimiles which are now tending toward digitization are subjected to a special study aimed at miniaturization, improvement in operating speed, and economization of energy. When the plate making is carried out by the use of such a thermal head, therefore, the stencil sheet to be used for the production of the plate is particularly required to exhibit high sensitivity and high resolution and possess a uniform quality.
As a means of producing the stencil sheet of this description, the method which comprises preparing a thin single-layer film, tightly pressing the film against a specular-surface roll, and superposing the film on a supporting member [Japanese Patent Application Disclosure SHO 60(1985)-89,396] has been proposed, for example. The method of this nature is at a disadvantage in requiring a meticulous care in the handling of the film, being complicate operationally, necessitating the use of an expensive laminating device and a clean plant atmosphere, and demanding a consideration for protection against adverse effects of static electricity. In this method, When the film is made of a material susceptible to fracture or a material of weak nerve (low modulus of elasticity) or the film has a very small thickness of less than 2 .mu.m the stencil sheet produced cannot acquire fully satisfactory quality because the film is prone by nature to various troubles.
Studies are also under way in search of a method capable of imparting enhanced sensitivity to the film itself which is destined to be perforated. For example, the method which comprises causing a soft polymer relatively susceptible to thermal fusion to be superposed by the natural solution casting method in the form of a very thin layer about 1.0 .mu.m in thickness on a porous supporting member, and quickly drying the superposed layer of polymer, thereby inducing cast orientation of the polymer and consequently producing a stencil sheet having the sensitivity of perforation improved owing to the smallness of thickness has been proposed [Japanese Patent Application Disclosure SHO 63(1988)-209,996]. The stencil sheet which is produced as described above, however, is unsatisfactory as in terms of the recovery of solvent and the speed of drying on account of the process of production and, moreover, is disadvantageous in inevitably losing locally the uniformity of thickness of the film and consequently having a restriction as to the lower limit of film thickness. Further, this method has no alternative but to use a film of low softening point because it is required to improve sensitivity in the state of low orientation. When the produced stencil sheet is preserved at a high temperature of about 50.degree. C., for example, it is liable to induce the phenomenon of curling and, in an extreme case, the phenomenon of delamination. When the plate making is carried out by the use of a thermal head, the stencil sheet is liable to stick to the thermal head. The stencil sheet is also disadvantageous in being deficient in resistance to the impact of printing and in resolution because it has conspicuously lower strength than the biaxially stretched film.
There are known methods which obtain a desired layer by costretching a multiplicity of layers including a supporting layer and then separating the layer from the other layers [Japanese Patent Application Disclosure SHO 57(1982)-176,125 and SHO 57(1982)-176,126]. These methods are unsatisfactory in that the produced film is deficient in stretching property and peeling property. These inventions do not suggest the high-sensitivity thermosensitive multilayer film of the present invention permitting easy separation and excelling in resistance to the phenomenon of curling. The method which comprises costretching a multilayer film combining a polyethylene terephthalate resin and a polypropylene resin, then heat-treating the costretched film at an elevated temperature in the range of 100.degree. to 240.degree. C., for example, thereby thoroughly crystallizing the polyethylene terephthalate layer and imparting enhanced peeling property to the layer, and subsequently peeling and obtaining a film resistance to heat (useful as a thin-wall film for a condenser, for example) has been known to the art [Japanese Patent Application Disclosure SHO 60(1985)-178,031]. The method which comprises subjecting the film of a similar layer combination to sequential biaxial stretching and then simultaneous biaxial stretching, subsequently heat-treating the stretched film at a temperature higher than the crystal melting point of polypropylene and lower than the crystal melting point of polyester, thereby sufficiently promoting the crystallization of the polyester, and thereafter effecting the peeling has been known [Japanese Patent Application Disclosure SHO 61(1986)-31,236]. These methods are invariably based on the concept that since the peeling made during the course of stretching results in fracture of the film or in loss of uniformity of the film, the facility of peeling ought to be ensured by thoroughly carrying out the heat treatment mainly after the stretching thereby promoting the crystallization of polyester. These methods have been utilized in the method which comprises first laminating the film and a supporting element and subsequently carrying out the peeling [Japanese Patent Application Disclosure SHO 63(1988)-53,097] and the method which comprises similarly laminating a multilayer film and a supporting member and subsequently carrying out the peeling thereby producing a stencil sheet for plate making [Japanese Patent Application Disclosure SHO 64-14,092]. All of these methods belong to a technical level different from the technical level of the present invention which concerns the method for highly efficient production of a high-sensitivity film and a stencil sheet for plate making enjoying high facility of peeling and high quality and the method for producing a stencil sheet for plate making excelling in resistance to the phenomenon of curling.
The present inventors have filed their basic invention relating to the high-sensitivity themosensitive film with the Japanese Patent Office [Japanese Patent Application Disclosure SHO 62(1987)-282,983] and with the U.S. Patent and Trademark Office [U.S. Pat. No. 4,766,033]. This invention is directed to the thermosensitive film itself and is highly effective in itself. When the stretched multilayer film is peeled to obtain the desired layer and this layer is laminated with the porous supporting member, the lamination at times is difficult to perform and the produced laminate is liable to curl.